Id look at extrusion width. So one way to do this is extrude some filament into the air. Let it cool then measure it with calipers. For a 0.4mm nozzle typically it will be 0.48mm diameter due to die swell. For a 0.3mm expect 0.36mm. If so great as the default extrusion width in most slicers is 1.2x nozzle diameter. If your filament in free air is bigger set a wider extrusion width.
Also make sure set your slicer to print inside perimeters first or anything overhanging gets printed in mid air.